In a wireless communication system or network, devices may transmit over a wireless channel only after sensing that the channel is not in use (“clear” or “idle”). However, if the devices try to transmit immediately after sensing that the channel is not currently in use, all the devices that were waiting for a clear channel may try to transmit at the same time immediately after the channel ceases to be busy. The resulting “collision” between the signals can prevent one or more of the devices from making a successful transmission.
To reduce the chance of such collisions, some wireless communication standards implement a contention-based mechanism, in which a “Contention Window” (CW) scheme includes a contention period, during which devices that want to transmit will wait, after sensing an open channel, before actually performing a transmission. According to this contention-based mechanism, each device may choose a time period (“the back-off period”), e.g., randomly, and wait until the channel has been idle for this time period before trying to transmit (“first transmission attempt”). The CW defines the maximum period that the device should wait, e.g., the random values are chosen to be within the CW. If the resulting first transmission attempt is unsuccessful, the length of the contention window can be repeatedly increased, e.g., doubled, for subsequent retries, up to some maximum value, until a retry is successful, or until a maximal number of retransmissions is reached. The CW period may be defined by a first value, denoted CWmin, which defines a minimum starting size of the CW, and a second value, denoted CWmax, which defines the maximal size of the CW.